The present invention relates generally to a reactor core coolant flow rate control system for a boiling water reactor type nuclear power plant in which internal pumps (recirculation pumps) are adopted for forcively feeding or recirculating a coolant or cooling water through a reactor vessel containing a core including a fuel assembly for thereby generating a steam to rotate a steam turbine driving an electric generator.
As typical examples of the conventional reactor core coolant flow rate control system for the boiling water reactor type nuclear power plant (hereinafter referred to as the BWR type nuclear power plant) in which internal pumps are employed, there can be mentioned those disclosed, for example, in JP-A-3-77959 (hereinafter referred to as the literature #1), JP-A-5-18396 (hereinafter referred to as the literature #2) and a drawing FIG. 9, 4-3 annexed to "WRITTEN APPLICATION FOR ALTERATION OF KARIBA NUCLEAR POWER PLANT REACTOR INSTALLATION IN KASHIWAZAKI, JAPAN" (hereinafter referred to as the literature #3). As can be seen in FIG. 1 of the accompanying drawings, the reactor core coolant flow rate control which is system disclosed in the literature #3 is comprised of ten recirculation or internal pumps 3-1 to 3-10 provided in association with ten variable-frequency power supply units (inverter units) 423A-1 to 423A-5 and 423B-6 to 423B-10, respectively, four input transformers 421A, 422A, 421B and 422B, two AC motors 403A and 403B, two AC generators 405A and 405B, two plant or house transformers 20-1 and 20-2, and circuit breakers 412A, 424A, 412B and 424B. For convenience of the description, a combination of the motor 403A and the generator 405A as well as a combination of the motor 403B and the generator 405B will hereinafter simply be referred to as the motor/generator sets, respectively. Furthermore, a reference numeral 7 denotes an AC generator driven by a steam turbine which is rotating under the work of a steam generated within a BWR (nor shown).
The output power generated by the turbine-driven generator 7 undergoes a voltage transformation through is transformed by the house transformers 20-1 and 20-2 to be supplied to the motor/generator sets (403A/405A and 403B/405B) by way of the motor/generator set trip circuit breakers 412A and 412B, respectively, and to the input transformers 422A and 421B via the associated circuit breakers 424A and 424B, wherein the outputs from the motor/generator sets (403A/405A and 403B/405B) are inputted to the input transformers 421A and 422B, respectively. The powers outputted from the input transformers 421A, 422A, 421B and 422B are inputted to the inverter units 423A-1 to 423A-5 and 423B-6 to 423B-10, respectively, which serve to convert the frequency of the power supplied to the internal pumps 3-1 to 3-10, respectively, for controlling the rotation speeds (rpm) of these internal pumps.
By taking advantage of the fact that each of the motor/generator sets (403A/405A and 403B/405B) has a mechanical inertia, such an arrangement is adopted that even when the power supply for the reactor core coolant flow rate control system is subjected to external disturbance due to occurrence of a failure or fault in the power transmission system of the nuclear power plant, the power supply to the inverter units 423A-1 to 423A-5 and 423B-6 to 423B-10 can be maintained for a predetermined period in succession to the occurrence of such disturbance, for thereby allowing the reactor core to be continuously cooled. At this juncture, it should however be noted that each of the motor/generator sets 403A/405A and 403B/405B itself has neither the function nor capability for converting the frequency of the power supplied to the internal pumps 3-1 to 3-3 and 3-8 to 3-10. The frequency conversion is preformed by the variable-frequency power source units 423A-1 to 423-3 and 423B-8 to 423B-10 which are constituted by the inverter units, respectively, as mentioned previously. Parenthetically, the inverter unit can exhibit substantially no mechanical inertia. The circuit breakers 412A, 424A, 412B and 424B are installed for the purpose of protection of the machines and units disposed downstream thereof.